In the last two decades, the availability of wide-field (i.e., \(\approx 10^3-10^4\,\mathrm {deg^2}\) ) optical/near-infrared (NIR) surveys, such as the Sloan Digital Sky Survey (SDSS; e.g., [1]}), the UKIRT Infrared Deep Sky Survey (UKIDSS; e.g., [2]}), the Canada-France
High-redshift Quasar Survey (CFHQS; e.g., [3]}) and the Panoramic Survey
Telescope & Rapid Response System 1 (Pan\(-\) STARRS1; e.g., [4]}), led to the discovery of \(>200\)  quasars (QSOs) at \(z\gtrsim 6\) , when the Universe was \(\lesssim 900\)  Myr old. Currently, eight of these objects have been identified at \(z>7\)  [2]}, [6]}, [7]}, [8]}, [9]}, with the redshift record holder recently identified at \(z=7.642\) , during the reionisation era [10]}.
The relatively shallow coverage provided by wide field optical/NIR surveys implies that only high-redshift QSOs with the strongest rest-frame UV continuum emission can be detected, such that their selection is strongly biased towards rare (about 1 Gpc\(^{-3}\)  at \(z\sim 6\) ; [11]}) and luminous (\( \log \frac{L_{bol}}{L_{\odot }}\gtrsim 13\) , or \(M_{1450}\lesssim -24\) , where \(M_{1450}\)  is the QSO absolute magnitude at rest-frame 1450Å; Fig. REF ) systems. Only recently the Subaru High-z Exploration of Low-Luminosity Quasars project (SHELLQs; e.g., [8]}) discovered the first moderate-luminosity (down to \(M_{1450}\approx -22\) ) QSOs at \(z\gtrsim 6\) , which are thus still poorly
known in terms of basic demographics, birth, and growth.
